binary-tree-postorder-traversal

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    vector<int> postorderTraversal(TreeNode *root) {
        vector<int> res;
        if(!root) return res;
        
        stack<TreeNode*> s;
        TreeNode* curr = root;
        TreeNode* previsited = NULL;
        while(curr || !s.empty()){
            while(curr){
                s.push(curr);
                curr = curr->left;
            }
            curr = s.top();
            if(curr->right == NULL|| curr->right == previsited){
                res.push_back(curr->val);
                previsited = curr;
                s.pop();
                curr = NULL;
            }
            else
                curr = curr->right;
        }
        return res;
    }
};